Walled gardens refer to services where the service provider has control over its ecosystem, including applications, content, organic media, paid media and any gathered data. Examples of walled gardens include Google, Facebook and Amazon. These service providers have access to persistent consumer identification which enables them to manage control groups and run sound A/B tests when it comes to advertising campaigns. These tests can be used to measure the ROI of media investments made within an individual walled garden. Walled gardens constrain external marketers’ ability to measure media ROI, as they represent an additional measurement constraint.